The invention is directed generally to a process for stabilizing organic resist image layers and more particularly to the treatment of such layers with a fluorine containing plasma and then baking.
Many microimaging processes such as, for example, integrated circuit fabrication require high temperatures. However, the upper limit of the processing temperature is often restricted by the resist masking material properties. The deformation of the resist image during high temperature baking (and especially for temperatures greater than about 200.degree. C.) also places a constraint on the minimum image size of the resist mask pattern and hence the product geometry. Organic polymer resist image layers are conventionally baked after development in order to improve resist adhesion and the physical properties of the resist. Baking is necessary so that the resist will withstand the chemicals used to etch the substrate and/or high temperature treatment such as metal deposition without separating from the substrate or being subject to image distorting flow. The problem of flow is particularly important in microimaging processes such as integrated circuit manufacture where dimensions are becoming smaller as the technology advances and very fine lines must be preserved. The resist layer is baked to at least a temperature to which it is expected to be subjected during substrate processing. The baking process is itself limited to times and temperatures where image distortion due to flow is minimized. Where the permissible baking conditions are insufficient to provide the needed resist properties, then an alternate technique of improving resist stability must be used.
Previous techniques for improving resist image stability have included bombardment of the resist image by placing the image bearing substrate in a glow discharge on an electrode with an inert gas atmosphere such as argon, nitrogen, or mixtures of nitrogen and hydrogen as described in, for example, Defensive Publication No. T883,005, dated Feb. 2, 1971. Treatment of resist images in a glow discharge with oxygen is described for example in U.S. Pat. Nos. 3,816,196 and 3,920,483. Where the resist image bearing substrates are placed in a plasma on an electrode such as, for example in the process described in T883,005 and subjected to ion bombardment, by choosing the correct conditions, stability of the resist layers to high temperatures can be achieved. However, in an electrodeless plasma treatment with oxygen or an inert gas, it has been found that resist image stability to temperatures of only up to about 210.degree. C. can be provided. Where temperatures above this must be withstood, and it is undesirable or inconvenient to use a treatment involving ion bombardment of the resist layer and substrate, there was a need for an improved electrodeless plasma treatment. It has unexpectedly been found that a short electrodeless glow discharge treatment using a fluorine containing atmosphere permits the heating or baking of the resist layer to temperatures of at least about 330.degree. C. to thereby provide resist image stability during processing at very high temperatures.